Steady state and pre-steady state kinetic studies on the interaction of myosin subfragment-one with actin and ATP were carried out in a continuing effort to elucidate the steps which occur when actomyosin hydrolyzes ATP. Our results during the past year strongly suggest that the rate limiting step in the actomyosin ATPase cycle is not the hydrolysis of ATP in the initial Pi burst but is a subsequent slower step which we have called the transition from the refractory to the non-refractory state. At relatively low actin concentration this transition occurs with the myosin dissociated from the actin. On the other hand, we now find that at very high actin concentration all of the myosin-nucleotide complexes including the refractory state are able to bind to actin. Furthermore, both the hydrolysis of ATP in the initial Pi burst and the transition from the refractory to the non-refractory state can occur when the myosin is complexed with actin; and these steps occur at about the same rate when the myosin is attached to actin as when the myosin is dissociated from actin. These findings suggest a kinetic model for the actomyosin ATPase where there is no mandatory dissociation of the actomyosin complex during each cycle of ATP hydrolysis but rather there is a cycle from myosin states strongly attached to actin to myosin states weakly attached to actin.